1. Field of The Invention
The present invention relates generally to an automotive suspension control system. More particularly, the invention is directed to a variable damping force shock absorber control system which is operable to modify damping characteristics based on vibratory energy transmitted from a road surface to a vehicle body for assuring riding comfort.
2. Description of The Prior Art
U.S. Pat. No. 3,807,678 to Karnopp et al., issued on Apr. 30, 1974, discloses a semi-active suspension system which establishes damping characteristics substantially corresponding to those in so-called skyhook dampers. This prior art system is designed to control a shock absorber in a manner wherein when a sprung vertical speed d.times.2 and a relative speed d.times.2-d.times.1 between sprung and unsprung portions of a vehicle are in the same direction, that is, when a vehicle body and a wheel move in opposite directions or when the vehicle body moves faster than the wheel in the same direction, a damping force is increased, while when the sprung vertical speed d.times.2 and the relative speed d.times.2-d.times.1 in opposite directions, the damping force is decreased.
Therefore, in the Karnopp et al. system, when although the relative speed d.times.2-d.times.1 is in a negative direction (i.e., during a compression, or bound stroke of the shock absorber), the sprung vertical speed d.times.2 is in the negative direction (i.e., a downward direction), the shock absorber is controlled to assume a greater damping force.
Additionally, Japanese Patent First Publication No. 1-502972 teaches a shock absorber control system which utilizes a sprung acceleration sensor and a stroke sensor. The outputs of the acceleration sensor are integrated to determine a sprung vertical speed d.times.2. The outputs of the strike sensor are differentiated to derive a relative speed d.times.2-d.times.1 between sprung and unsprung portions of a vehicle. When a direction of the sprung vertical speed d.times.2 or the relative speed d.times.2-d.times.1 has been changed damping characteristics are modified.
Such a conventional shock absorber control system, however, has suffered from a drawback in that when vertical vibratory motion of the vehicle body is increased, a control valve is actuated to control the shock absorber to assume firm damping characteristics, therefore, when high-frequency vibrations due to fine protrusions on a road surface are input to the vehicle body, the actuation of the control valve is increased so that the high-frequency vibrations are undesirably transmitted to vehicle occupancies as a result of provision of the firm damping characteristics in favor of damping the vibratory motion of the vehicle body. Accordingly, a great deal of power is consumed and the durability of the control valve is reduced. In addition, it is difficult to,damp the sprung vibrations while reducing transmission of the vibrations to the vehicle body.